1. Field of the Invention
The present invention generally relates to a device for position measurement or angular measurement and more particularly relates to a device for position measurement in servo controlled mechanisms such as manipulators of industrial robots, wherein the angular position of synchros or resolvers at several measurement locations are converted into digital signals by time multiplexing.
2. Description of the Prior Art
Synchros have been used for measuring angles and providing for a remote read-off via electrical transfer of an angle signal from a measuring location to a indicating intrument. Synchros have also been used for the transfer of moments, usually small, and for the remote control of servo mechanisms.
When an angular measurement is desired to be converted into digital information, a resolver, which is a particular type of synchro is used. The resolver includes two stator windings which are shifted by 90.degree. and one or two rotor windings. The windings may be supplied in one of two ways with A.C. voltage at for example 400 Hz.
One method supplies A.C. voltage to the two stator windings with a voltage represented by E. sin .omega.t and E. cos .omega.t, respectively. There is then induced in the rotor windings a voltage of k.E. (sin .omega.t.sin .theta.+cos .omega.t.cos .theta.) which equals k.E.sin (.omega.t-.theta.) wherein k is a constant. Thus, a phase modulated signal with constant amplitude is created and may be converted into digital form by a demodulator.
A second method supplies A.C. voltage to the rotor winding of the resolver with a voltage value represented by E.sin .omega.t. There is then induced in the stator sine winding a voltage: k.E sin .omega.t.sin .theta., and in the stator cosine winding a voltage of k.E sin .omega.t.cos .theta.. This latter method is most frequently used today for conversion of the analog signal into digital signals.
Resolvers used for digital conversion of angular measurement are competitive with code disks which provide direct digital angular read-off information. The resolver-digital-converters are still preferred over code disks where great resolution is necessary, for example, greater than 10 bit per revolution.
Most of the available resolver-digital converters use a resolver having its rotor winding supplied with A.C. voltage as noted above. The most frequently used converter is the so-called "tracking"-converter wherein the analog sine and cosine signals from the resolver are multiplied by cosine and sine values, respectively, to the contents of an up/down counter. The output signals from the multipliers are subtracted and a modulated signal is derived representing the sine of the difference between the turning angle of the resolver and the contents of the counter.
The different signal is demodulated and through a voltage controlled oscillator provides counting pulses to the counter so that the loop is closed and the contents of the counter follows the resolver angle. Tracking converters provide high static accuracy and, are insensitive to frequency, phase and amplitude variations. Tracking converters are also insensitive to high frequency noise present on the resolver signals and provide good tracking of rapidly rotating resolvers having a substantially constant velocity and enable direct angular read-off measurements.
Another widely used construction is the so-called Successive-approximation-converter. It operates in a manner similar to the tracking converter, but instead of providing a continuous read-off of the resolver signals, the resolver signals are sampled once or twice per period. A DC voltage signal representing the sine of (.theta.-.phi.) is formed by a sine and cosine multiplication and subtraction and is applied to a quick operating analogue/digital converter which includes a Successive-approximation register. Successive-approximation converters enable rapid conversion, which permits relatively rapid changes of the resolver speed, and provides for a direct angular read-off.
A more recent construction for converting several channels is the micro-processor-based system of MN7200 Micro Networks wherein the operation for one typical channel demodulates sine and cosine signals (AC/DC) and converts such signals into digital form separately. The micro-processor thereby receives the sine and cosine values of the resolver angle.
The micro processor calculate tan .theta.=(sin .theta./cos .theta.) and thereafter .theta. is derived by an arc tan table and by interpolation. This system is somewhat expensive, since one AC/DC converter is required for each channel. Low rotational speed of, for example, of the resolver 10.degree./sec. is acceptable before accuracy is reduced.
The said system MN2700 provides great static accuracy, is insensitive to noise and is multiplexed for several channels.